This invention relates to optical waveguide transmission devices and more particularly to optical waveguide devices suitable, for example, for employment in optical multiplexer and demultiplexer, and router applications employing Wavelength Division Multiplexing (WDM) which may find application in optical fiber based communication networks.
Increasingly, state-of-the-art optical transmission devices for wavelength division multiplexing utilize waveguide gratings. In a typical device structure a waveguide grating may be placed between input and output slab waveguides. Multi-wavelength light from a point source, the end of the input waveguide, is transmitted through the grating and different individual wavelengths are focused on different points of the output waveguide surface. The grating itself may include a number of different physical length waveguides with a constant length increment. This results in a wavefront rotating with the wavelength change. The grating can be also used to focus an output waveform and to collimate an input waveform. For this purpose the ends of the grating may be located on a circular surface provided by each slab waveguide, at which the waveguides are located close to each other to assure strong coupling needed for high efficiency. However, in the grating itself, coupling between individual waveguides is not desirable, so that the waveguides are spaced relatively far apart from each other. This implies a relatively large grating structure. In an optical multiplexer, multi-wavelength light from a point source, the end of the input waveguide, is transmitted through the grating and different individual wavelengths are focused on different points of the output waveguide surface.
Close coupling between waveguides, at the interface between the slab waveguide and the grating, has typically been considered essential to the efficiency of the device. To alleviate concerns about resulting aberrations, it has been proposed to position the foci of interface arcs between the input waveguide and the slab region, and the slab region and the waveguide grating, some distances from the arcs themselves. This approach of positioning the focal points of the arc boundary (center of the interface arc) away from the opposite arc, originating from the desire to maintain strong coupling, has continued. The emphasis on strong coupling was consistent with grating based multiplexer designs for handling a small number of channels (for example, 1xc3x974 to 1xc3x978) and high efficiency, i.e. low transmission loss.
Another desideratum in multiplexer design is a compact device, and in particular a compact waveguide grating section. This is driven by cost reduction associated with reduced size, as well as lower sensitivity to material non-uniformities. One approach addressing the compactness issue has been to introduce a reflector in the grating section. Possible reflecting arrangements include waveguide Bragg reflectors and mirrors, including mirrors with stepped surfaces. However, such approaches have only partially addressed the overall issue of compactness and transmission efficiency.
According to the present invention, an optical waveguide grating, suitable for operation in an optical transmission device, comprises an array of laterally spaced grating waveguides each having an end extending from a free space region and another end terminating at a reflector surface. Neighboring ones of the grating waveguides differ in length from each other by a constant increment, preferably defined as an optical path length increment. The grating waveguides include curved portions having respective radii of curvature which decrease progressively according to the sequential location of the grating waveguides commencing from a reference grating waveguide.
Preferably, the radii of curvature of said curved portions of the grating waveguides decrease towards each opposite side of the array of grating waveguides from one or more reference grating waveguides at or near the center of said array.
Advantageously, the radii of curvature of said curved portions of the grating waveguides decrease in an approximately parabolic manner as the numerical sequential location of a curved portion increases from the central grading waveguide location to respective locations at opposite sides of the grating waveguide array. The rate of sequential decrease in the radii of curvature of said curved portions may be modified to result in preferential attenuation of optical signal reflection at said curved portions located near the edges of the grating waveguide array to implement apodization.
According to a different aspect of the invention, a waveguide grating, suitable for operation in an optical transmission device, comprises an array of laterally spaced grating waveguides, wherein the grating waveguides are laterally spaced apart and each grating waveguide has one end extending from a free space region. Neighboring ones of the grating waveguides differ in optical path length from each other by a constant optical path length increment. The grating waveguides include curved portions having respective radii of curvature which decrease progressively according to the sequential location of the grating waveguides commencing from a reference grating waveguide, preferably located at or close to the center region of the array of grating waveguides.
By implementation of features of the invention, an optical waveguide device can be designed which is particularly compact. For example the overall length of the optical waveguide grating readily can be designed to be less than the overall length of the free space region, both measured along the general direction of transmission of light wave signals.
Structures embodying the invention may be used to implement a WDM device capable of handling a larger number of channels (e.g. 40 channels with a channel separation as close as 50-100 GHz)while giving rise to low cross talk between adjacent channels (e.g. less than 50 db) and significantly reduced variation of transmission efficiency across the response spectrum of the device. An important contribution to the improved functionality device is the decoupling of the waveguide sections along the length of the waveguide grating, and in particular at the ends of the waveguide sections at the waveguide slab end. The particular mode of deployment of the tapered waveguide sections described above significantly contributes to this desired separation without adversely affecting overall transmission efficiency.